1. Field of the Invention
The present invention relates to a photo-mask used for a semiconductor device and a manufacturing method thereof, and a registration accuracy enhancement method thereof.
2. Description of the Background Art
A registration accuracy measurement mark has conventionally been used for measuring registration accuracy of each layer in a semiconductor device manufacturing process. The registration accuracy measurement mark is usually formed in several layers, that is, in each of the stacked inherent layers of the device which are the object of actual registration accuracy measurement, at a predetermined position on a semiconductor substrate. When measuring registration accuracy, the positions of stepped portions, i.e. edges of the registration accuracy measurement mark are detected, and the distance between the two detected stepped portions, i.e., edges is measured, whereby the amount of displacement of the registration between the registration accuracy measurement marks in two layers is calculated. The difference between the amount of displacement of the registration of the two opening patterns in the inherent layers formed on a photo-mask and the calculated amount of displacement of the two stepped portions of the registration accuracy measurement marks is thereby measured as a registration error in a manufacturing process.
Though the above-described stepped portions of the registration accuracy measurement mark are detected using a registration accuracy measurement device, the registration error itself cannot be accurately measured because the displacement of the pattern, i.e., transfer error, caused by lens aberration is not considered.
A method of measuring registration accuracy considering a lens aberration is described in Japanese Patent Laying-Open No. 9-74063. In the technology described in the laid-open application, registration accuracy measurement marks are formed in two layers in the same shape as two inherent layers to be measured for actual registration accuracy, so that the amount of the pattern displacement, i.e., transfer error, of the two inherent layers due to aberration and that of the registration accuracy measurement marks due to aberration come to be the same. The pattern displacement of the inherent patterns caused by aberration and that of the registration accuracy measurement marks caused by aberration are thereby offset. Therefore, only the registration error is detected in the registration accuracy measurement method described in Japanese Patent Laying-Open No. 9-74063.
In a registration accuracy measurement method using only a conventionally used registration accuracy measurement mark, however, the pattern displacement itself caused by lens aberration is not calculated to modify a lens unit (hereinafter, the modification of a lens unit means both the adjustment of the lens unit and the correction of a lens) so as to reduce the aberration. Thus, the registration error caused by lens aberration is not corrected to enhance the registration accuracy in a semiconductor device manufacturing process.
The following method, however, is generally used to evaluate the amount of aberration. First, a resist film 100, provided with a pattern in which lines and spaces are alternately formed, is formed as shown in FIGS. 7 and 8. The line-widths of L1 and L2 at the opposite ends of resist film 100 are measured, for example, using SEM (Scanning Electron Microscope). The line-width abnormal value due to comatic aberration is generally calculated by the following equation:Line-width abnormal value=(L1−L2)/(L1+L2)  (1) 
From the equation (1), the line-width abnormal value can be calculated according to the measurement results of the above-described widths L1 and L2.
In this method, only a pattern in which lines and spaces are alternately formed is used for resist film 100, so that only the measurement error of the pattern line-width caused by aberration can be detected when using normal SEM. That is, a pattern transfer error resulting from the difference in shape between two patterns to be measured due to the aberration can not be detected. Thus, in manufacturing a semiconductor device, a lens unit which is modified to reduce the transfer error of the pattern caused by lens aberration has not been used.
Furthermore, since SEM is used, the measurement time is long and the measurement of the whole lens unit (the entire exposure device) is difficult.